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In opentk-0.9.7.7z/Build/Instructions.txt, the following statement is found:
Build instructions
If you have Visual Studio 2005/2008 or MonoDevelop 2 (post-beta1), simply open OpenTK.sln in the Build/ folder. If you wish build from the commandline, install nant (http://nant.sourceforge.net) and execute Build.exe in the Build/ folder.

I cannot find any .sln file in the Build/ folder of the .zip, nor in the Source/-subfolders.

Also, referring to "the Build/ folder" is ambigous, since there are several folders called "Build" in opentk-0.9.7.7z.